This invention relates to a rack and pinion gear and to a method of assembling such a gear and is primarily concerned with rack and pinion gears as are employed in motor vehicle steering systems.
Conventionally a rack and pinion gear for vehicle steering has a rack bar mounted in a pinion housing to be displaceable along its longitudinal axis. Rotatably mounted in the housing is a pinion the teeth of which engage with the teeth of a rack on the rack bar so that rotation of the pinion effects in the required displacement of the rack bar. In the majority of applications, and for ease of manufacture, the rack teeth are substantially co-planar and parallel to the longitudinal axis of the rack bar whereby it is intended that as the pinion rolls over the rack bar the spacing between the pinion axis and the longitudinal axis of the rack bar is maintained constant. It is also conventional to provide in the pinion housing support means such as a yoke or slipper which is located oppositely to the position of engagement between the rack and pinion to slidably engage the rack bar on the side thereof remote from the rack and to resiliently bias the rack into engagement with the pinion as the rack bar is displaced over the operative length of its rack which, for vehicle steering, usually corresponds to three to six revolutions of the pinion.
It is important, particularly for vehicle steering purposes, that the rack bar is borne by the support means to exhibit smooth displacement within the pinion housing while sufficient force is exerted by the support means to maintain the rack and pinion in engagement (bearing in mind the forces which will be exerted between the engaging teeth and which tend to move those teeth apart during normal use of the gear. For economy of manufacture on a large scale as is usual for rack and pinion steering gears, the tolerances to which the individual components of a rack and pinion assembly can be manufactured are such that with a gear of the kind discussed above, the spacing between the pinion axis and the longitudinal axis of the rack bar is not maintained constant as the rack bar is driven over the operative length of the rack. Accordingly, when the rack bar is not subjected to a steering load the support means is likely to be displaced against, and under the influence of, its resilient biasing as the rack bar is driven within the pinion housing so that the "tightness" of the rack bar (that is the resistance which it experiences to its longitudinal displacement in the housing) can vary in accordance with the changes in the biasing force applied by the support means. The above-mentioned displacement of the support means may be attributed, partly to pemissible tolerances in the manufacture of the pinion, which tolerances may permit the pinion to rotate about an eccentric axis so that as the pinion rotates the spacing between its axis and the longitudinal axis of the rack bar changes and partly to permissible tolerances in the rack bar itself and in the rack teeth formed in the rack bar. Because of the difference in characteristics caused by permitted tolerances in manufacture, which will vary from one rack and pinion gear to another, it is standard practice that each gear when assembled has its support means adjusted to apply the maximum biasing force to urge the rack and pinion into engagement when the rack bar is at its tightest position in the housing, that is with the rack bar located in the housing so that the pinion axis and the rack axis are at their maximum spacing. The aforementioned maximum biasing force is of course predetermined according to the particular gear characteristics. Conventional practice therefore in the rack and pinion steering gear industry is that a gear, having been assembled, is finally adjusted by rotating the pinion to move over the operative length of the rack; determining the position of maximum tightness of the rack bar in the housing, and at that position adjusting the pressure biasing applied by the support means to the maximum force at which it is intended the rack should be urged into engagement with the pinion when the rack bar is unloaded. By such a procedure it will be apparent that as the unloaded rack bar is driven within the pinion housing the biasing force from the support means should never be greater than that originally intended and the possibility of the rack bar becoming "too tight" in the housing is alleviated. It will be realised that when the rack bar is loaded during its use for steering purposes, the biasing force exerted from the support means may be considerably greater than that to which it is adjusted when the bar is unloaded and this is taken into consideration, as is well known in the art, during the adjustment and depending upon the characteristics of the support means.
From the aforegoing it will be seen that in conventionally constructed rack and pinion gear assembly of the kind above referred to, the final adjustment of the gear to determine the appropriate resilient biasing provided by the support means can be a time consuming process which adds considerably to the manufacturing costs and is usually regarded as an inconvenience. It is an object of the present invention to provide a rack and pinion gear and a method of assembling such a gear by which the aforegoing disadvantages can be alleviated.